Accumulated overtime are hours that you work that is outside of your normal work day. For example if you work from 8 Am - 4 Pm, then working from 4 Pm - 6 Pm will become your overtime hours, which means that you have 2 hours of overtime. For these extra hours at work, you are paid an extra fee and these hours are registered in the balance in overtime. These which can be used as time off work.
Flexitime are hours that are gained through work, but these do not give you any extra money for overtime, however when you register flexitime hours you can later on use these hours as time off work.